The present invention relates to an electroluminescent device having a luminescent layer comprising a luminescent material, which is capable of directly converting the energy of electric field applied thereto into a light energy and capable of providing a large surface-area luminescence different from conventional incandescent lamp, fluorescent lamp, light-emitting diode, etc.
The electroluminescent device is classified into the following two groups depending on the luminescent exciting mechanism.
(1) An AC driving-type electroluminescent device of exciting luminescent material by local transfer of electrons or holes in a luminescent layer and emitting light only under an AC electric field. PA0 (2) A DC driving-type electroluminescent device of exciting luminescent material by injection of electrons and holes from electrodes and re-combination of them in a luminescent layer and actuated under a DC electric field. PA0 2,5-bis(5,7-di-t-benzyl-2-benzoxazolyl)-1,3,4-thiadiazole; PA0 4,4'-bis(5,7-di-t-benzyl-2-benzoxazolyl)stilbene; PA0 2,5-bis(5,7-di-t-benzyl-2-benzoxazolyl)thiophene; PA0 2,2'-(p-phenylenedivinylene)-bis-benzothiazole; PA0 4,4'-bis (2-benzoxazolyl)biphenyl; PA0 2,5-bis(5(.alpha.,.alpha.-dimethylbenzyl)-2-benzoxazolyl)thiophene; PA0 4,4'-bis(5,7-di-(2-methyl-2-butyl)-2-benzoxazolyl)stilbene, and 2,5-bis(5,7-di-(2-methyl-2-butyl)-2-benzoxazolyl)-3,4-diphenylthione. As the hole transporting compound, there are disclosed, PA0 1,1-bis(4-di-p-tolylaminophenyl)-4-phenyl-cyclohexane, PA0 1,1-bis(4-di-p-tolylaminophenyl)-cyclohexane; PA0 4,4"-bis(diphenylamino)-quadoriphenyl, PA0 (4-dimethylamino-2-methylphenyl)phenylmethane; and N,N,N-tri(p-tolyl)amine.
The AC driving-type electroluminescent device uses a luminescent material composed of an inorganic compound comprising ZnS and Mn, Cu, etc. added thereto, but it involves various drawbacks, for example, that it requires high AC electric field of higher than 200V for driving, needs expensive production cost and shows insufficient luminance and durability.
In the DC driving-type electroluminescent device, high luminance can be obtained since a thin film organic compound has been used as the luminescent layer.
Japanese Patent Application Laid-Open (KOKAI) No. 59-194393, U.S. Pat. No. 4,539,507, Japanese Patent Application Laid-Open (KOKAI) No. 63-295695, U.S. Pat. No. 4,729,432 and Japanese Patent Application Laid-Open (KOKAI) No. 63-264692 disclose electroluminescent devices each comprising an anode, an organic hole injecting and transporting zone, an organic electron injecting luminescent zone and a cathode. As the typical materials used therefor, there can be mentioned, for example, an aromatic tertiary amine as the organic hole injecting and transporting material, and aluminum trisoxine as the organic electron injecting luminescent material.
For instance, Japanese Patent Application Laid-Open (KOKAI) No. 59-194393 (U.S Pat. No. 4,539,507) discloses an electroluminescent device comprising an anode, a hole transporting zone, an organic luminescent zone and a cathode, in which the total film thickness of the anode, the hole transporting zone and the organic luminescent zone is not greater than 1 .mu.m, and at least one of the anode and the cathode allows to permeate at least 80% of radiation rays at a wavelength of not lower than 400 nm, and has at least 9.times.10.sup.-5 W/W of electric power conversion efficiency. As the fluorescent whitening agent of the electron transporting compound for forming the organic luminescent zone, there are used, for example,
U.S. Pat. No. 4,720,432 discloses an electroluminescent device comprising in sequence, an anode, an organic hole injecting and transporting zone, an organic electron injecting and transporting zone and a cathode, characterized in that the organic hole injecting and transporting zone is composed of
a layer in contact with the anode, containing a hole injecting porphyrinic compound and
a layer containing a hole transporting aromatic ternary amine interposed between the hole injecting layer and the electron injecting and transporting zone.
Further, Jpn. Journal of Applied Physics, vol. 27, p. 713-715 reports an electrolumiscent device comprising an anode, an organic hole transporting layer, a luminescent layer and an organic electron transporting layer and a cathode. Referring to the material used, there are mentioned, for example, N,N-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine as the organic hole transporting material, 3,4,9,10-pentylene tetracarboxylic acid bis benzimidazole as the organic electron transporting material and phthaloperylene as the luminescent material.
These examples show a necessity for studying various characteristics of these organic compounds and effectively combining such characteristics into an electroluminescent device for using such organic compounds as the hole transporting material, luminescent material or electron transporting material. In other word, it is necessary to research and develop organic compounds in a wide range.
However, study for the DC driving electroluminescent device using the organic compounds including the foregoing embodiments as the luminescent material has only a short history and it can not be said that the study for the material and the application thereof to the device has been made sufficiently. Accordingly, there are various problems at present with respect to a further improvement for the luminance, a requirement for more versatile emission wavelength so that luminescent hues such as blue, green and red can be selected precisely in a case of application to a full color display and an improvement for durability.
An offer of an electroluminescent device capable of providing versatile emission wavelength, and giving various luminescent hues and also excellent in durability is strongly demanded.
As a result of the present inventors' earnest studies on the constituent elements of the luminescent layer for overcoming the foregoing problems, it has been found that the foregoing technical problems can be solved by an electroluminescent device comprising an anode, a cathode and one or plurality of organic compound layers sandwiched therebetween, wherein at least one of the said organic compound layers comprises an organic compound represented by the following general formula (II) or (III) as the constituent ingredient: ##STR1## wherein R.sup.1 and R.sup.2 represent respectively a substituted or unsubstituted alkyl group, a substituted or unsubstituted carbocyclic aromatic ring, or a substituted or unsubstituted heterocyclic aromatic ring, R.sup.1 and R.sup.2 being possibly joined together to form a ring.
Ar.sup.1 and Ar.sup.2 represent respectively a substituted or unsubstituted carbocyclic aromatic ring, or a substituted or unsubstituted heterocyclic aromatic ring, and n represents an integer of 1, 2 or 3; ##STR2## wherein X represents --CH.sub.2 CH.sub.2 --, --CH.dbd.CH--, --O--, --S-- or ##STR3## R.sup.1, R.sup.2, R.sup.3, R.sup.4 and R.sup.5 represent respectively a substituted or unsubstituted alkyl group, a substituted or unsubstituted carbocyclic aromatic ring, or a substituted or unsubstituted heterocyclic aromatic ring, R.sup.1 and R.sup.2 being possibly joined together to form a ring, and PA1 Ar.sup.1 represents a substituted or unsubstituted carbocyclic aromatic ring, or a substituted or unsubstituted heterocyclic aromatic ring. PA1 Ar.sup.1 represents a substituted or unsubstituted carbocyclic aromatic ring, or a substituted or unsubstituted (5-13 membered) heterocyclic aromatic ring, B represents --CH.dbd.CH--.sub.n Ar.sup.2 or ##STR5## (wherein X represents --CH.sub.2 CH.sub.2 --, --CH.dbd.CH--, --O--, --S-- or ##STR6## R.sup.3, R.sup.4 and R.sup.5 represent respectively a substituted or unsubstituted (C.sub.1 -C.sub.18) alkyl group, a substituted or unsubstituted carbocyclic aromatic ring, or a substituted or unsubstituted (6-membered) heterocyclic aromatic ring, Ar.sup.2 represent a substituted or unsubstituted carbocyclic aromatic ring, or a substituted Or unsubstituted (5-13 membered) heterocyclic aromatic ring, and n represents an integer of 1, 2 or 3).
The present invention has been attained based on this finding.